The invention relates generally to a low magnitude AC and DC current sensor, and more particularly to such devices employing magnetoresistive sensors.
One of the most important electrical safety devices is a Ground Fault Circuit Interrupter (GFCI). GFCIs are designed to provide protection against electrical shock from ground faults, which occur when the electrical current in a circuit, either a wire line or an appliance, strays outside the path where it should normally flow. This ground fault, or unintentional electric path between a source of current and a grounded member, occurs when current is leaking somewherexe2x80x94in effect, electricity is escaping to ground. If a human body provides a path to ground for this leakage the person could be burned, severely shocked or electrocuted. Such a condition may be a shock hazard, even when the current flow is insufficient to trip an electrical circuit breaker associated with the current flow.
Modern world-wide electrical codes require that certain circuits in electrical wiring systems likely to be in contact with moisture include current interrupting devices which are designed to protect the user against shock by interrupting power when a current leakage is initially detected. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation in bathrooms, kitchens, spas, garages and outdoors. GFCI devices enjoy widespread use in many countries around the world. GFCI devices are sometimes called Earth Leakage Circuit Breakers (ELCB) or Earth Leakage Switches as well as Residual Current Circuit Breakers (RCCB) or Residual Current Devices (RCD).
Conventional GFCI devices that are designed to trip in response to the detection of a ground fault condition typically employ one of two methods. In one approach a first current transformer senses the circuit line current and a second transformer senses the circuit neutral current and a comparison circuit is used to determine whether there is a remainder current as indicative of a ground fault. Another approach uses a summing transformer to surround both the line and neutral conductors and determines the presence of a ground fault when the resultant current is below a predetermined value. In either case, when such an imbalance is detected, a circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the load.
Around the world the applications for GFCI""s involve a wide variety of conditions. For example, in the United States a ground fault current in excess of 6 milliamperes cannot be permitted. However, in other countries the permissible ground fault current may be as high as 30 milliamperes. Accordingly, a GFCI for use in all international situations must be able to provide protection against ground fault currents in the range of 6-30 milliamperes.
Also, not all countries utilize 60 hertz AC power that is utilized in the United States. Therefore, a GFCI for international applications must be able to provide protection for a frequency range of 50-60 hertz. Further, this GFCI will operate in applications requiring other AC frequencies such as 400-Hz, which is the standard electrical system operating frequency of commercial aircraft. In come situations the GFCI must be able to respond to pulsating DC requirements. GFCIs in the art do not presently meet all of these requirements in a satisfactory manner.
GFCI devices using current transformers (xe2x80x9cCTxe2x80x9d) cannot sense DC current in power circuits for the reason that CTs only respond to AC current. At DC (zero) frequency, the output of a CT is zero so that a circuit incorporating a CT as a current measuring device has a 100% error. Even at frequencies of 30 Hz, prior art CT devices have a significant error in current measurement. A DC component in the AC mains will also cause a CT error.
An attempt to overcome prior art shortcomings is found in U.S. Pat. No. 5,986,444 to Powell. Powell teaches a device for detecting low magnitude electrical currents that may include leakage currents. A generally toroidally-shaped member made of magnetic material provides an air gap and a magnetoresistive device is located in the air gap for sensing a current flowing through a conductor that passes through the toroidal member. Magnetoresistive devices are resistive elements typically arranged in a Wheatstone or balanced bridge arrangement that changes resistance value in the presence of a magnetic field. In order to reduce damage due to overcurrents, the member has a portion of reduced cross-sectional area to cause saturation of the member. The apparatus measures the variation of magnetic field strength acting in the magnetoresistive sensor as a measure of current faults.
Saturated mode magnetoresistive sensors are popularly used as rotation speed sensors by detecting the existing magnetic flux bending when a gear, made of high permeability material like steel, is rotating nearby the surface. For example, see U.S. Pat. No. 6,194,893 to M. Yokotani et al. A permanent magnet is put under one side of the magnetoresistive sensor. When gear teeth fly by the other side of the sensor nearby, the unevenness of the gear surface causes a magnetic field to change its direction back and forth, which results in the change of resistance valus of magnetoresistor, causing a voltage signal to be generated.
Other patents of interest include U.S. Pat. No. 5,933,306 which use GMR sensors for use in a GFCI device. U.S. Pat. No. 5,923,514 shows use of a GM device within the gap of a toroid to measure magnetic field strength. U.S. Pat. No. 5,461,308 shows use of a GMR device in the airgap of a magnetic material for sensitive current measurement.
An object of the invention is to provide a reliable low current sensor without susceptibility to stray external magnetic fields, susceptibility to undesired saturation of the magnetic member due to current surges, and inability to measure a DC component while still providing protection to the consumer from hazardous leakage currents at all frequencies.
The above object has been met with a new current sensor that can be used in a frequency independent ground fault detector that substantially increases sensitivity, adjustability and reliability for low level leakage tripping. The new current sensing approach involves forcing magnetic field lines induced by an unbalanced portion of a circuit into a sensitive region of a magnetoresistive sensor. The unbalanced portion of the circuit is associated with a current fault in a pair of wires that are part of a current loop. The current fault is manifest due to magnetic flux lines in paired windings about a toroidal magnetic member, with non-cancelling magnetic flux lines, associated with the current fault, protruding from a gap in the toroidal magnetic member. At the same time, magnetic flux lines emerge from a nearby permanent magnet, with the two sets of flux lines permeating each other in a magnetic flux line mesh zone existing between the permanent magnet and the toroidal member. The magnetoresistive sensor has a sensitive region, which is normally planar, placed in the flux line mesh zone. Within the magnetic flux line mesh zone, the permanent magnet bends flux lines from the toroidal member into the sensitive plane of the sensor. By operating the magnetoresistive sensor in this manner, current through the sensor varies with changes in the balanced to unbalanced states of the toroidal member. Stability of operation and immunity to external electrical noise is promoted. The current signal produced by the sensor representing the unbalanced state is amplified, filtered and transmitted to a circuit interruption trip solenoid.